'use strict';
var Buffer = require('buffer').Buffer;
var charTable = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567';
var byteTable = [
  0xff, 0xff, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
  0xff, 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06,
  0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e,
  0x0f, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16,
  0x17, 0x18, 0x19, 0xff, 0xff, 0xff, 0xff, 0xff,
  0xff, 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06,
  0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e,
  0x0f, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16,
  0x17, 0x18, 0x19, 0xff, 0xff, 0xff, 0xff, 0xff
];

function quintetCount(buff) {
  var quintets = Math.floor(buff.length / 5);
  return buff.length % 5 === 0 ? quintets : quintets + 1;
}

exports.encode = function (plain) {
  if (!Buffer.isBuffer(plain)) {
    plain = new Buffer(plain);
  }
  var i = 0;
  var j = 0;
  var shiftIndex = 0;
  var digit = 0;
  var encoded = new Buffer(quintetCount(plain) * 8);

  /* byte by byte isn't as pretty as quintet by quintet but tests a bit
      faster. will have to revisit. */
  while (i < plain.length) {
    var current = plain[i];

    if (shiftIndex > 3) {
      digit = current & (0xff >> shiftIndex);
      shiftIndex = (shiftIndex + 5) % 8;
      digit = (digit << shiftIndex) | ((i + 1 < plain.length) ?
        plain[i + 1] : 0) >> (8 - shiftIndex);
      i++;
    } else {
      digit = (current >> (8 - (shiftIndex + 5))) & 0x1f;
      shiftIndex = (shiftIndex + 5) % 8;
      if (shiftIndex === 0) i++;
    }

    encoded[j] = charTable.charCodeAt(digit);
    j++;
  }

  for (i = j; i < encoded.length; i++) {
    encoded[i] = 0x3d; //'='.charCodeAt(0)
  }

  return encoded;
};

exports.decode = function (encoded) {
  var shiftIndex = 0;
  var plainDigit = 0;
  var plainChar;
  var plainPos = 0;
  if (!Buffer.isBuffer(encoded)) {
    encoded = new Buffer(encoded);
  }
  var decoded = new Buffer(Math.ceil(encoded.length * 5 / 8));

  /* byte by byte isn't as pretty as octet by octet but tests a bit
      faster. will have to revisit. */
  for (var i = 0; i < encoded.length; i++) {
    if (encoded[i] === 0x3d) { //'='
      break;
    }

    var encodedByte = encoded[i] - 0x30;

    if (encodedByte < byteTable.length) {
      plainDigit = byteTable[encodedByte];

      if (shiftIndex <= 3) {
        shiftIndex = (shiftIndex + 5) % 8;

        if (shiftIndex === 0) {
          plainChar |= plainDigit;
          decoded[plainPos] = plainChar;
          plainPos++;
          plainChar = 0;
        } else {
          plainChar |= 0xff & (plainDigit << (8 - shiftIndex));
        }
      } else {
        shiftIndex = (shiftIndex + 5) % 8;
        plainChar |= 0xff & (plainDigit >>> shiftIndex);
        decoded[plainPos] = plainChar;
        plainPos++;

        plainChar = 0xff & (plainDigit << (8 - shiftIndex));
      }
    } else {
      throw new Error('Invalid input - it is not base32 encoded string');
    }
  }

  return decoded.slice(0, plainPos);
};
